1. Field of the Invention
The present invention relates to the rotating head mechanism of a rotary type AC magnetic flux leakage flaw detector.
2. Description of the Prior Art
Regarding wire rods, steel bars, etc. which include a material of comparatively small diameter having the maximum diameter of 30 mm or 40 mm by way of example, a material stretched with a die by the use of a wire stretcher, a material drawn with a die as in the wire stretcher by a combined machine, and so on; the grain of the outer surface of the material is good so as to exhibit a surface roughness of 5-10 S (a ruggedness of 5-10 .mu.m), the piece of the material to-be-tested has a favorable circularity and has a small diametrical deviation, and the dimensional accuracy of the material is high in such a manner that the magnitude of a bend in the lengthwise direction of the material is 1-2 mm/m. As to such a piece to-be-tested, accordingly, the centering between a rotating probe and the test piece is easy, and the distance between the rotating probe and the outer surface of the test piece is easily maintained within a predetermined allowable range. Therefore, a satisfactory flaw detection accuracy can be guaranteed even with the automatic eddy-current flaw detection which employs the rotating probe. It is known, however, that, in general, all the factors of the surface grain behavior, the circularity and the bend of the test piece worsen more as the outer diameter of the test piece becomes larger. By way of example, a steel bar whose outer diameter is about 50-60 mm is manufactured in such a way that hot-rolled steel left intact is cooled and that the bend is merely corrected by a straightening machine. The roughnesses of the outer surfaces of some of such steel bars reach 15 S-300 S or 500 S (15 .mu.m-300 .mu.m or 500 .mu.m). Besides, the bent magnitude per unit length is large and sometimes reaches 2-3 mm/m or 3-4 mm/m. In order to automatically detect dotty flaws in the surface of such a test piece, a linear flaw extending in the rolled direction thereof, etc., a rotary flaw detector which rotates a plurality of probes and which can advance the test piece straightforward to obtain helical flaw detection traces can be used as in the foregoing case. Since, however, the dimensional accuracy of the test piece is not favorable in contrast to the foregoing case, there is the problem that a mechanism for the centering between the test piece and the rotating probes needs to be contrived. Also, there is the problem that the grain property of the surface of the test piece is not applicable to the eddy-current flaw detection of the prior art based on the rotating probes. More specifically, the mechanical dimensional accuracy is inferior for such a reason that the hot-rolled surface left intact is uneven and has iron oxide chips (scales) adhering thereto. Besides, the inhomogeneous state of the electromagnetic surface behavior renders it difficult to detect linear flaws (seams, hair cracks and longitudinal cracks) in mass formation which flaws have an opening of a small width by the prior-art eddy-current flaw detection means.
Therefore, various technical improvements have heretofore been made for the detections of the surface flaws of the hot-rolled steel materials of inferior surface grain behaviors. Among them, a magnetic flux leakage flaw detection method is known as replacing fluorescent magnetic powder flaw detection based on visual inspection. As means for magnetizing the steel piece in the flux leakage flaw detection, there are known a D.C. flux leakage, an A.C. magnetic flux leakage, and the combination of them. Here, the D.C. flux leakage method has the merit that both the flaws of the inner surface and outer surface of a steel tube or the like can be detected, but it is problematic in the following points:
(1) During a flaw detection operation, the attractive force of a D.C. electromagnetic exerts an evil effect to spoil the mechanical centering between the tube being inspected and exciting magnetic poles.
(2) As a higher flaw detection accuracy is intended, the value of ampere-turns to be applied to the D.C. electromagnet must be increased more, with the result that the exciting magnetic poles and the electromagnet overheat due to Joule heat.
(3) The flaw detection accuracy is limited to a flaw depth of about 0.3 mm-0.2 mm as the lower limit value, and it has been empirically known that the method cannot meet the demands of industrial circles of which the detection of still shallower flaws is required.
On the other hand, the conventional A.C. magnetic flux leakage method usually employs a magnetosensitive device, for example, a Sony magnetodiode or a Hall effect device. Such a device is subject to a restricted frequency response characteristic inherent in a semiconductor, the upper limit of an exciting frequency to be applied to the device is 2 kHz or 3 kHz, and the flaw detection accuracy of the device in terms of the detection capability thereof is limited to a flaw depth of about 0.3 mm-0.2 mm. As an expedient for overcoming this limit of the detection capability, it has heretofore been proposed that a search coil (sensor probe) of small diameter is employed as a detecting device corresponding to frequencies for wide applications, whereupon the exciting frequency to be applied is set as high as 4-16 kHz. Thus, the depth of magnetization directly under the surface of a test piece is reduced by the skin effect, whereby excitation energy is focused into the bounds of a depth required for flaw detection, and a high flux density is established by the focusing. As a result, an AC magnetic flux leakage from a minute flaw part is increased to enhance the detection capability.
Although the magnetic flux leakage flaw detection method is more suited to the flaw detection of the hot-rolled steel material or the like of inferior surface grain behavior than the eddy-current flaw detection method, it has the large number of problems to-be-solved as stated before. Particularly in a rotary type flux leakage flaw detector, exciting magnetic pole portions and a group of detecting probes need to be held in a predetermined mechanical positional relationship. Moreover, unless the group of detecting probes and the exciting magnetic pole portions are kept withdrawn during rotation till the arrival of the fore end of the steel material being the test piece, they might be damaged by the fore end of the steel material. Accordingly, the rotary type flux leakage flaw detector necessitates a mechanical coupling/interlocking setup which can hold the exciting magnetic pole portions and the group of detecting probes in the predetermined mechanical positional relationship and can withdraw them from the test piece on the necessary occasion.
As such mechanical coupling/interlocking setups, there have hitherto been proposed ones wherein, using an electromagnet or a rotary solenoid, the detecting probes are made free to retract and are thus prevented from damaging, as disclosed in the specification of U.S. Pat. No. 3,299,350, the specification of U.S. Pat. No. 3,612,987, and the official gazette of Japanese Patent Application Publication No. 48-36916. In addition, the official gazette of Japanese Patent Application Publication No. 51-44675 discloses a setup wherein exciting coils and detecting devices are mounted on a rotating disc and wherein pinch rollers adapted to be pneumatically operated are arranged before and behind the disc in the passing direction of a test piece so as to restrain the test piece, thereby intending to detect a flaw stably. Further, the specification of U.S. Pat. No. 4,297,636 discloses a setup wherein a yoke lever and a probe lever are disposed indendently of a magnet yoke and a probe support and wherein the yoke lever is set to a closed position by a fore-end bent portion provided in a stopper plate, thereby making it possible to protect the magnet yoke from any hindrance in the movement of the yoke lever in the opening direction thereof. With this setup, the probe lever is installed by the turning pin of a pin holding plate which is fixed to a part of the yoke lever, and a counterweight is mounted on the opposite side to probes with respect to the fulcrum.
Any of the rotating head mechanisms of the rotary type flux leakage flaw detectors hitherto proposed as mentioned above is such that exciting magnetic pole portions and a group of detecting probes are fixedly mounted on a rotating disc. Therefore, in case of detecting the flaw of a test piece of different outer diameter, such a troublesome operation has been involved that the exciting magnetic pole portions and the group of detecting probes are adjusted so as to be appropriate for the outer diameter or that they are replaced with others appropriate for the outer diameter.
An object of the present invention is to provide the rotating head mechanism of a rotary type leakge-flux flaw detector which can eliminate the problems of the prior-art techniques as stated before.